Purpose: The aim of this study was to compare the delineation and treatment planning of two phase- based (the end-expiration and the end-inspiration phases) IGTV (Internal Gross Tumor Volume) with ten-phase based (4D) IGTV.

Methods: Two patients with lung tumors at different sites were selected for the study.The location of the tumor in Patient A was at the upper lobe(attached to the chest wall) of the left lung.In patient B the location was the middle lobe of right lung.We contoured the GTV on each of the ten respiratory phases of the 4D-CT data set.The combination of these GTVs produced the IGTV .GTV was also generated on the extreme respiratory phases 0% phase = peak inhalation, 50% phase = peak exhalation).The combination of these two GTVs produced IGTV <2Phases>.Treatment planning was done and dose to OARs were compared in both end expiration inspiration(EEI) and 4DCT study set.

Results: IGTV <2Phases> (175cc)and IGTV (180 cc) for Patient A were nearly same. But for Patient B, IGTV <2Phases> (80cc) was smaller than the IGTV (181 cc).C.I.(Conformity Index) was found better in EEI and 4DCT in Patient A than in Patient B due to the limitation of the AAA algorithm in the low density region.Lung-GTV doses were less in EEI scan than in 4DCT for Patient B whereas it was the same for EEI and 4DCT in Patient A.

Conclusion: The study shows the patients with tumor in upper lobe have no difference in tumor coverage and OARs sparing in the EEI and 4DCT. However,tumors in middle lobe have a greater excursion during respiration and hence greater all phase IGTV.We suggest performing a 4D scan and EEI scan both,for these categories of patients in order to include the tumor excursion due to breathing